1. Field of the Invention
The present invention relates to an electronic double-layer capacitor, and a process for producing the same.
2. Description of the Related Art
An electric double-layer capacitor is conventionally put to practical use as a capacitor in which a high capacity on the order of farad (F) can be provided without being accompanied by a chemical reaction and which is strong to the charging and discharging of large electric current and to a charging/discharging cycle.
In recent years, a new application of such an electric double-layer capacitor as an auxiliary power source or the like of a small-sized electronic device such as a mobile phone and the like by utilizing the feature of the electric double-layer capacitor. A further reduction in size and a further increase in capacity of the electric double-layer capacitor are required.
Such an electric double-layer capacitor includes a basic cell comprising a pair of collectors, a separator disposed between the collectors, and a pair of polarizable electrodes each of which is disposed between each of the collectors and the separator and in which an electrolytic solution is contained. The collectors provided at opposite ends of a laminated-cell structure made by laminating the basic cells or the collectors of the basic cells as connections in series are electrically connected to electrode plates included in lead terminal/electrode plate assemblies, so that they are charged from and discharged to the outside.
The capacity of the cell structure can be changed easily by changing the number of the basic cells connected in series and hence, the electric double-layer capacitor is comprised of the laminated-cell structure in many cases rather than the single basic cell.
The basic cell can be produced using a producing process disclosed, for example, in U.S. Pat. No. 6,377,441 by Ohya and others.
Among the components of the basic cell, the collector has functions to encapsulate the electrolytic solution and to provide an electric conductivity, and may be a conductive film formed from an insulating resin and a conductive material in many cases. In general, materials disclosed in PCT/JP98/01021 by Tsukakoshi and others and in PCT/JP98/03073 by Akita and others can be used as a material suitable for the collector. The thickness of the collector is about 0.2 mm, but in recent years, a thinner film having a thickness of several tens xcexcm has been started to be used in order to meet the need for a reduction in size of the components.
The use of the electric double-layer capacitor as a variety of small-sized electronic devices such as an auxiliary power source for a mobile phone and the like is expected, but an electric double-layer capacitor existing presently suffers from the following two problems in respect of equivalent series resistance (which will be referred to as ESR hereinafter) of a product, i.e., an initial inferiority of ESR and a deterioration in ESR with the passage of time.
(1) There are many products having an initial ESR larger than a practical level. The ESR value is dispersed largely among products, resulting in a degraded yield.
(2) If a product having a small initial ESR has been put in service for a long period, its ESR is increased remarkably and for this reason, such product cannot be put in service (a long-term reliability is poor).
In a case of a thin collector film, particularly, the deterioration in ESR with the passage of time is significant, which is an obstacle to the real employment of the electric double-layer capacitor to an application expected.
The capacitor is used in wider applications such as an integrated circuit, a backup power source, a coupling circuit, a high-frequency circuit and the like, but the ESR is an important factor governs the overall characteristic in each of the applications. Therefore, it is desired that the ESR value is extremely small and stabilized. Particularly, when the electric double-layer capacitor is used in an application as a power source, a larger capacity and a lower ESR value are considered as ideal.
The ESR is governed mainly by a loss resistance of a dielectric material in a lower frequency range and mainly by a resistance of the component itself constituting the capacitor and a resistance between the components in a higher frequency range. What attention is paid to is particularly factors associated in the higher frequency range, which include
(a) a resistance of the collector itself (material),
(b) a resistance of the electrode plate itself (material),
(c) a state of contact between the basic cells (between the collectors), and
(d) a state of contact between the outermost collector of the cell structure and the electrode plate.
Among them, particularly, (c) and (d) are issues of consideration.
Accordingly, it is a first object of the present invention to provide an electric double-layer capacitor and a process for producing the same, wherein the problems associated with the above-described conventional electric double-layer capacitor can be solved, and a lower ESR value can be realized at an initial stage and after the service of the capacitor for a long period.
It is a particular object of the present invention to provide an electric double-layer capacitor and a process for producing the same, wherein the state of contact between the basic cells is good at the initial stage after the manufacture and after the service of the capacitor for a long period.
It is another particular object of the present invention to provide an electric double-layer capacitor comprising a single basic cell and a process for producing the same, wherein the state of contact between the collector and the electrode plate included in the lead terminal/electrode plate assembly is good at the initial stage after the manufacture and after the service of the capacitor for a long period.
To achieve the above objects, according to a first aspect and feature of the present invention, there is provided an electric double-layer capacitor comprising a cell structure constituted by laminating basic cells in series with their collectors serving as connections, each of the basic cells including a pair of collectors, a separator disposed between the collectors, and a pair of polarizable electrodes which are disposed between the collectors and the separator and in which an electrolytic solution is contained; and lead terminal/electrode plate assemblies each having an electrode plate electrically connected to the collectors at opposite ends of the cell structure, each of the collectors including a matrix made of an elastomer having a hardness Hs in a range of about 55 (inclusive) to lower than about 85 at ambient temperature, and conductive particles dispersed in the matrix.
With such configuration, the hardness of the elastomer included in the collector is in the above-described range and hence, the collector is difficult to deform by the pressure of a transmission gas resulting from the gasification of the electrolytic solution, and it is difficult to create voids in an interface between both of the collectors. Therefore, the state of contact between the basic cells is good at the initial stage after the manufacture and after the service of the capacitor.
According to another aspect and feature of the present invention, there is provided an electric double-layer capacitor, comprising a basic cell including includes a pair of collectors, a separator disposed between the collectors, and a pair of polarizable electrodes which are disposed between said collectors and said separator and in which an electrolytic solution is contained; and lead terminal/electrode plate assemblies each including an electrode plate electrically connected to each of the collectors of the basic cell; the collector including a matrix formed of an elastomer having a hardness Hs in a range of about 55 (inclusive) to lower than 85 at ambient temperature, and conductive particles dispersed in the matrix; and a conductive bond layer being interposed between each of the electrode plates and each of the collectors electrically connected to the electrode plate, the conductive bond layer including a matrix formed of an elastomer having a hardness Hs in a range of about 60 (inclusive) to lower than 85 at ambient temperature, and conductive particles dispersed in the matrix.
With the above configuration, since the collector and the conductive bond layer have the above-described configures, the conductive bond layer is difficult to deform by the pressure of the transmission gas resulting from the gasification of the electrolytic solution, and it is difficult to create voids in an interface between the collector and the conductive bond layer. Therefore, the state of contact between the outermost collector of the cell structure and the electrode plate is good at the initial stage after the manufacture and after the service of the capacitor.
According to a further aspect and feature of the present invention, there is provided a process for producing an electric double-layer capacitor, comprising the steps of laminating a plurality of basic cells in series one on another with their collectors serving as connections, each of the basic cells including a pair of collectors, a separator disposed between both of the collectors, and a pair of polarizable electrodes which are disposed between the collectors and the separator and in which an electrolytic solution is contained; forming conductive bond layers on surfaces of electrode plates of a pair of lead terminal/electrode plate assemblies; bringing the conductive bond layers on the electrode plates and the collectors forming opposite ends of the laminate of the basic cells into abutment against each other; and pressing the laminate so that the pair of electrode plates are clamped.
In the electric double-layer capacitor produced by the producing process having such feature, voids are difficult to create in an interface between both of the collectors and an interface between the collector and the conductive bond layer, and the capacitor has a good ESR characteristic at an initial stage and even after the service for a long period.
According to a yet further aspect and feature of the present invention, there is provided a process for producing electric double-layer capacitor, comprising the steps of forming conductive bond layers on surfaces of electrode plates of a pair of lead terminal/electrode plate assemblies; bringing the conductive bond layers formed on the surfaces of the electrode plates into abutment against surfaces of a basic cell outside the collectors; and pressing the resulting laminate so that the electrode plates are clamped.
In the electric double-layer capacitor produced by the producing process having such feature, voids are difficult to create in an interface an interface between the collector and the conductive bond layer, and the capacitor has a good ESR characteristic at an initial stage and even after the service for a long period.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.